The present invention relates to apparatus for conditioning fibrous materials, and more particularly to apparatus for drying tobacco. Still more particularly, the invention relates to improvements in apparatus for reducing the moisture content of successive increments of a continuous tobacco stream.
It is already known to reduce the moisture content of tobacco in several stages, namely, during transport of a continuous tobacco stream through successive drying units in each of which the particles of tobacco are contacted by a hot fluid medium. Tobacco which is heated in such apparatus may consist of shreds (comminuted tobacco leaf laminae), ribs or particles which are converted into the fillers of cigars or cigarillos. The drying action must be predictable with a high degree of accuracy because, in the manufacture of cigarettes and similar rod-shaped smokers' products, the deviation of moisture content of tobacco from an optimum moisture content cannot exceed a fraction of one percent. Each and every deviation of moisture content from an optimum moisture content exerts a highly undesirable influence upon the machinery in which a continuous rod-like filler of tobacco is draped into a web of cigarette paper or the like as well as on the quality of the ultimate product.
U.S. Pat. No. 3,259,995 to Powischill discloses a method and apparatus for drying tobacco in several stages. The temperature of the fluid which contacts tobacco in the first drying unit is ascertained for the purpose of adjusting the temperature of fluid in the second unit when the monitored temperature deviates from a desired value. A drawback of such apparatus is that the temperature of fluid in the first unit is not a reliable indicator of the moisture content of tobacco which advances toward the second unit. Therefore, the final moisture content often deviates from the desired final moisture content. Furthermore, the patented apparatus is rather complex in spite of the fact that it cannot establish predictable conditions for drying in the first unit.